In recent years, the use of prefabricated, modular building panels which are factory formed then taken to the building site and assembled has become increasingly in demand. Such building panels generally take the form of a pair of spaced walls or skins having inserted therebetween some kind of insulating core such as expanded plastics, cork, wood and the like. A honeycomb section has also been used, however, this more for structural support than insulation. Primarily in recent years, cork and wood have been replaced by the expanded plastics which function quite satisfactorily because they are rigid and self-supporting, as well as being good insulators. In certain situations, however, expanded plastics may form fire hazards as is the case in oven walls.
In such situations it is more desirable to use a fire retardant or fire suppressing material which is also a good insulator. Fiberglass is one such material; however, it suffers from the disadvantage that it is very soft and not structurally sound. It will not support and retain the spaced skins or panel walls in their properly spaced position. Therefore, unless the skin itself is to be made stiffer (and necessarily more expensive), so that the edges, when joined, will be structurally sound, some other type of stiffening means is required particularly along the edge where the two panels are joined.